Michael Williams Michael Jercinovic

Exploring new frontiers in X-Ray microanalysis: The Ultrachron trace element microprobe and high resolution in-situ geochronology

Michael Williams

Michael Jercinovic

Geosciences

Geochronology – traditionally using isotopic/mass-spectrometric techniques• IDTIMS• Ion Probe

Electron Microprobe (EPMA)• High spatial resolution

access to ultra-thin rims,micro-domains, and inclusions

• In-situ: relate composition (and age) tomicro/macro-structure and mineral paragenesis

• Non-destructive• Integrated spatial / compositional / age relationships

Monazite: LREE-phosphate with Th and U (→ radiogenic Pb)Common accessory phase in many rocksFabric formerDissolution/re-precipitation reactions result in polygenetic

nature, and ties into overall reaction history

ThMα

Dating events{

2081232

232teTh

Pb

20619928.0238

238teU 20710072.0

235

235

teU

Radiogenic Pb accumulates as a function of Th and U decay constants and time…

ionconcentratPb )( ppmPb

ionconcentratTh )( ppmTh

ionconcentratU )( ppmU

)(yearsage

232 = 232Th decay constant (4.95E-11/yr)

238 = 238U decay constant (1.55E-10/yr)

235 = 235U decay constant (9.85E-10/yr)

Schematic model for evolution of felsic granulites in retrograde shear zones. B. Schematic illustration of corresponding monazite growth.

Ultrachron performance – analyses of standards

UMass Ultrachron:y = 1.007x - 8.6712

R2 = 0.9999

0

500

1000

1500

2000

2500

3000

0 500 1000 1500 2000 2500 3000

TIMS age (Ma)

EP

MA

ag

e (

Ma

)

Ultracrhron

Linear (1:1)

Detection limit:

3 ppm Pb, 4 ppm U achieved in 14 pt analysis (15min each)

The Ultrachron Project

Electron optics• Optimize analytical resolution

(Smaller phase analysis) for a range of kV and current

• High, stable current for trace element analysis

• Minimize excitation volume in high Z material

Detection

BSE and X-Ray optics• Improve precision (Optimize

counting - PbMα)

• Integrate spectrometers

• Improve accuracy – background estimation

Techniques• Minimize beam damage

• Background

• Analytical protocols

Improve dynamic range of BSE amplifier

BSE shielding for high current applications

New high intensity crystals (VLPET) + VL detectors

Counters optimized (gas mixture, pressure, HV)

Completely dry vacuum system

Anticontamination

CeB6

New HV power supply

Decouple operation of condensers to optimize brightness down column

Current regulation up to 1 microamp

VLPET development

dimensions (mm)W L area (mm2) area/PET

PET 18 29 522 1.00LPET 22 60 1320 2.53VLPET 30 80 2400 4.60

Crystal Comparison

0

100

200

300

400

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

length / PET

Inte

nsi

ty (

cps/

nA

)

PbMa

UMb

PET

LPET

VLPET

Improvement in Precision - GSC 8153 monazite SX50

GSC 8153 monazite SX-Ultrachron

PET LPET VLPET VLPET LPET new VLPETPbMa pyromorphite sp1 sp2 sp3 sp4 sp5 sp4

71.08 69.46 106.2 104.2 55.16 96.4987.33 71.5 98.05 96.01 65.37 74.56

FWHM 158.41 140.96 204.25 200.21 120.53 171.050.001584 0.00141 0.0020425 0.0020021 0.001205 0.0017105

λ 0.013861 0.012334 0.017871875 0.017518375 0.010546 0.014966875

GSC 8153 Pb region

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

54500 55500 56500 57500 58500 59500 60500 61500 62500 63500 64500

wavelength (sin-theta)

inte

ns

ity

(c

ps

/nA

)

Old VLPET SP3

New VLPET

Th M4-N3 Pb M4-O2 Pb Mβ

Th Mζ1

Pb Mα

Th Mζ2

Ce Lα (2)Pr Lα (2)La Lβ (2) La Lα (2)

UMβ UMβ42472 42472

VLPET new VLPETsp3 sp4

112 68118 93

FWHM 230 1610.0023 0.00161

λ 0.020125 0.0140875

Pk counts (cps) 19230 22625% count increase 18

% resolution improvement 30

ThMα ThMα47301 47301


120 93125 106

FWHM 245 1990.00245 0.00199

λ 0.0214375 0.0174125



PbMα PbMα60421 60421


124 79120 87

FWHM 244 1660.00244 0.00166

λ 0.02135 0.014525



Pb Ma Pb Ma260.3102 218.9275

0.357 0.3160.11 0.12

0.817 0.7235265.166 4430.286

34.073 30.719103184.5 87108.38

20.1 20.1

Pb Ma Pb Ma259.8425 275.3302

0.213 0.2290.156 0.1510.486 0.524

5252.746 5554.83337.7 28.95

102946 108752.820.07 20.07

X-RayPk-Bg Mean (cps/nA)Std.Dev (%)Theo.Dev (%)3 Sigma (Wt%)Pk Mean (cps)Bg Mean (cps)Raw cts Mean (cts)Beam (nA)

New VLPET comparison

Measured intensity PbMα on pyromorphite

% count rate improvementvs SP3 vs old SP4

5.96 25.76

sp3 sp4 sp3 sp4

GSC 8153 Pb region

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

54500 55500 56500 57500 58500 59500 60500 61500 62500 63500 64500


inte

ns

ity

(c

ps

/nA

)

Old VLPET (SP3)

New VLPET

GSC 8153 old VLPET (SP3)

GSC 8153 new VLPET

10% improvement in pk/bkg

Interference corrections

Interferences on PbMα

old VLPET new VLPET

Predicted* Measured reduced relative

CF CF CF intensity (%)

ThMζ 0.33 0.41 0.36 12

YLγ 0.70 0.88 0.82 7

LaLα 0.14 0.18 0.02 89*Based on increased count rate

Measurement issues:

Interferences

Pb region (PET) - Elk Mt.

Th M1Pb M4-O2

Pb M

Pb M5-O3 PbM

Au M

Au M3-N4

Y L2,3Th M1-O3 (2) Th M4-N3

Th M2-O4 (2)

Th M2

La L1 (2)

La L2 (2)

La L1 (2)

La L4 (2)La L

3 (2)Ce L

Ce L1 (2) Ce L

2 (2)

Pr L1 (2)

Pr L2 (2)

Sm Ll (2)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

55000 56000 57000 58000 59000 60000 61000 62000 63000 64000

Wavelength (sin-)

I (cp

s/n

A)

Bkg 2

Bkg 1 Upper Bkg

Natural Monazite

U region (PET) vs. ThLGG 246-5 m1 near K-feldspar

Th M4-O2

Th M5-P3

Th-M5 edge

Th M3-N4ThM

Th-M4 edge

UM

Th sM

Th sM3-N4

Th M4-P2

K sK4

K sK K K

1

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

40400 40900 41400 41900 42400 42900 43400 43900 44400Wavelength (sin-

)

I (c

ps/

nA

)

Ar-K edge

Interference effects

The case of mutual interference of first order lines

U Mregion, PET

Th MTh M4-O2

Th M5-P3

Th-M5 edge

Th M3-N4

Th M

Th-M4 edge

Th M2-N1U MU M

Ar-K edge

Ca K1,2

Ca s K

Th sM

Th sM

Th sM3-N4

Th M4-P2

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

37500 38500 39500 40500 41500 42500 43500 44500 45500 46500

Wavelength (sin-)

I (cp

s/n

A)

ThO2

Brabantite

Th interferences on U-M region

Measurement issues:

Background Curvature

Effect on net intensity of PbMα

GSC 8153 (VLPET)

0.80

1.30

1.80

2.30

2.80

3.30

3.80

4.30

4.80

54000 56000 58000 60000 62000 64000


inte

ns

ity

(c

ps

/nA

)

Pb Mα

GSC 8153 Pb region

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

54500 55500 56500 57500 58500 59500 60500 61500 62500 63500 64500


inte

ns

ity

(c

ps

/nA

)

Old VLPET (SP3)

Pb Mα

How to measure background?


Th M1Pb M4-O2

Pb M

Pb M5-O3 PbM

Au M

Au M3-N4

Y L2,3Th M1-O3 (2) Th M4-N3

Th M2-O4 (2)

Th M2

La L1 (2)

La L2 (2)

La L1 (2)

La L4 (2)La L

3 (2)Ce L

Ce L1 (2) Ce L

2 (2)

Pr L1 (2)

Pr L2 (2)

Sm Ll (2)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

55000 56000 57000 58000 59000 60000 61000 62000 63000 64000

Wavelength (sin-)

I (cp

s/n

A)

Bkg 2

Bkg 1 Upper Bkg

Natural Monazite

GdPO4 Pb region (PET)sp3

y = 8.520777E+01e -7.237583E-05x

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

54000 56000 58000 60000 62000 64000


inte

ns

ity

(c

ps

/nA

)

VLPET

Expon. (VLPET)

Linear (Series2)

PbMα

U region on NdPO4 (LPET)

y = 82.098e-0.0001x

0.7

0.9

1.1

1.3

1.5

1.7

1.9

37300 38300 39300 40300 41300 42300 43300 44300 45300


inte

ns

ity

(c

ps

/nA

)

UMβ

Ar K edge

PbMα

sp3

y = 8.520777E+01e -7.237583E-05x

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

54000 56000 58000 60000 62000 64000


inte

ns

ity

(c

ps

/nA

)

VLPET

Series2

PET SX50

Expon. (VLPET)

Expon. (PET SX50)

sp3

y = 8.520777E+01e-7.237583E-05x

0.8

1.3

1.8

2.3

2.8

3.3

54000 56000 58000 60000 62000 64000


inte

ns

ity

(c

ps

/nA

)

VLPET

Pb Ma

Series4

Expon. (VLPET)

Linear (Series2)

GSC 8153 scan vs GdPO4

UMass sp3 GdPO4 and GSC 8153 (VLPET)

y = 8.520777E+01e-7.237583E-05x

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

54000 56000 58000 60000 62000 64000


inte

ns

ity

(c

ps

/nA

)

VLPET

Series2

Pb Ma

GSC 8153 mzt

Expon. (VLPET)

Linear (Series2)

PbMα


Th M1Pb M4-O2

Pb M

Pb M5-O3 PbM

Au M

Au M3-N4

Y L2,3Th M1-O3 (2) Th M4-N3

Th M2-O4 (2)

Th M2

La L1 (2)

La L2 (2)

La L1 (2)

La L4 (2)La L

3 (2)Ce L

Ce L1 (2) Ce L

2 (2)

Pr L1 (2)

Pr L2 (2)

Sm Ll (2)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

55000 56000 57000 58000 59000 60000 61000 62000 63000 64000

Wavelength (sin-)

I (cp

s/n

A)

Bkg 2

Bkg 1 Upper Bkg


PbM

0.18

0.20

0.22

0.24

0.26

0.28

58500 59000 59500 60000 60500 61000 61500 62000

Wavelength (sin-)

I (c

ps

/nA

)

regressed backgroundexponential fit

backgroundtwo point, linear

Pb M background value two point, linear fitPos. 1 Pos. 2

Pb M background value, regressed - exponential fit

Bkg.pos. 1

Bkg.pos. 2

y = 0.679x-1

0

2

4

6

8

10

12

14

0.00 0.10 0.20 0.30 0.40 0.50 0.60

net intensity (cps/nA)

% e

rro

r o

f n

et in

ten

sity

Measured

Theoretical based on run5

At ~1000ppm Pb, 10% error can easily produce an age error of 35-40Ma (5 wt.% Th, 4000ppm U)

Becomes 50% error at ~ 0.015 net intensity

bkg net intensity (Pk-bkg)actual bkg 0.23544 0.059155lin fit 1 0.24223 0.052365diff 0.00679 -0.00679%error 2.883962 11.47832





Pb

0

2000

4000

6000

8000

10000

12000

14000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

net intensity (Pk-bkg cps/nA)

pp

m

Pb concentration vs. net intensity

Black Hills, Leg 8

Y Th Pb U Age (Ma) NElk Mt. fitted Ave 6494 119215 8453 3586 1395.2 15

bkg. SStd 105 2172 141 218 15.3StdErr 27 561 36 56 4

Elk Mt. linear Ave 6462 119184 8052 3560 1333 15bkg. SStd 120 2218 174 228 16.8

Pos. 1 StdErr 31 573 45 59 4.3

Elk Mt. linear Ave 6462 119184 8279 3560 1368.9 15bkg. SStd 120 2218 136 228 15.2

Pos. 2 StdErr 31 573 35 59 3.9

RT87-17 fitted Ave 19196 48043 1094 3984 400.2 5bkg. SStd 3518 3043 119 1343 8

StdErr 1573 1361 53 601 3.6

RT87-17 linear Ave 19194 48007 996 3958 365.4 5bkg. SStd 3519 3039 123 1340 10.1

StdErr 1574 1359 55 599 4.5

Concentration (ppm)

Note: The IDTIMS 207Pb/206Pb age for the Elk Mt. monazite is 1394+/-1 Ma (J. Baldwin and S. Bowring, unpublished data). The IDTIMS age for RT87-17 is 394+/-1Ma (Tucker, et al., 1990).

Measurement issues:

Counting linearity

Calibrate at low current, analyze at high current

SP3 1-12-04After picommeter adjustment

0.97

0.98

0.99

1

1.01

1.02

0 50 100 150 200 250 300

current (nA)

I / I 1

5

Initial

Initial

After adjustment

After adjustment

Pk Int (calc) vs. pk. cps

10nA

15nA20nA

25nA

30nA 35nA

45nA

80nA

50nA

60nA70nA40nA

200nA

175nA

150nA

125nA

100nA

90nA

y = -0.0005x + 59.249R2 = 0.9873

54.0

55.0

56.0

57.0

58.0

59.0

60.0

0 2000 4000 6000 8000 10000 12000

cps

c/s

/nA

Meas (5 u-sec)

DTim=8u-sec(predicted)

adjusted current(+1.5%)

Series4

Linear (Series4)

line (meas) vs. current

10nA

15nA

20nA25nA

30nA

35nA

45nA

80nA

50nA60nA

70nA

40nA

200nA175nA

150nA125nA100nA

90nA

56.5

57.0

57.5

58.0

58.5

59.0

59.5

60.0

60.5

0 50 100 150 200 250

current (nA)

c/s

/nA

Meas (5 u-sec)

DT 8u-sec(predicted)adj current +1.5%

meas. at 8 usecDTmeas. at 8 usecDT (nA corr)

Inte

nsi

ty

current

Inte

nsi

ty

current

Adjust dead-time

(corrected ≠ imposed)

Inte

nsi

ty

current

Adjust linearity

(current cut-off specified)

Measurement issues

Fluorescence interference

REE-L lines will fluoresce K Kα

K-feldspar or mica hosted monazite

Fluorescence range

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 5 10 15 20 25 30 35

distance (m)

I/I(0

)

La L Ce L Nd L

Fluorescence intensity (If/If(0))

X-Ray range

U region (PET) vs. ThLGG 246-5 m1 near K-feldspar

Th M4-O2

Th M5-P3

Th-M5 edge

Th M3-N4ThM

Th-M4 edge

UM

Th sM

Th sM3-N4

Th M4-P2

K sK4

K sK K K

1

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

40400 40900 41400 41900 42400 42900 43400 43900 44400Wavelength (sin-

)

I (c

ps/

nA

)

Ar-K edge

Interference effects

The case of mutual interference of first order lines

K fluorescence effect on U concentration

Apparent K concentration in monazite

0

100

200

300

400

500

600

700

800

900

1000

0 5 10 15 20 25 30 35 40 45

distance from edge (m)

K (

pp

m)

Adjacent to Ksp

Adjacent to qtz

qtz

Ksp

20 um

KspUTh

qtz

K fluorescence effect on apparent age

Effect of K fluorescence - LGG246-5 monazite

1450

1500

1550

1600

1650

1700

1750

0 20 40 60 80

distance from edge (m)

Ap

pa

ren

t E

MP

A A

ge

(M

a)

Adjacent to Ksp

Adjacent to qtz

High Th domainLow Th domain

1681 +/-1 Ma TIMS

Th U Ksp

qtz 20 um

Ksp

qtz

Documents

Michael Williams Michael Jercinovic